Cu2S-based superior anode for sodium-ion batteries

ABSTRACT

As anode having an anode material, a current collector, a graphene-based material, and the graphene-based material covers the anode material.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 62/964,530 filed Jan. 22, 2020, which is incorporated herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

Not applicable.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not applicable.

BACKGROUND OF THE INVENTION

There has been an ever-increasing demand on renewable clean energies (e.g., solar energy and wind energy) given that fossil fuels (i.e., petroleum, coal, and natural gas) are quickly depleting and causing severe environmental issues. However, these renewable clean energies can only operate intermittently, and their wide implementation needs electrical energy storage (EES) systems.

In this regard, sodium-ion batteries (SIBs), as an alternative technology to lithium-ion batteries (LIBs), are very promising to constitute large-scale EES devices for sustaining smart grids, due to the low cost and natural abundance of sodium. A challenge in SIBs lies in developing high-performance anodes, enabling high capacity, high-rate capability, and long-term cyclability.

Studies on sodium energy storage systems started before the 1980s, almost at the same time that lithium storage systems were developed. SIBs have an electrochemistry similar to LIBs. Although SIB cathodes (e.g., NaMnO₂ and NaFePO₄) have been developed successfully, SIB anodes were not satisfactory. Compared to Li-ions having a radius of 0.76 Å, Na-ions have a larger radius of 1.02 Å. This has made some LIB intercalation-based anodes not suitable for SIBs. For instance, fewer Na-ions can be intercalated into graphite, which is a widely commercialized LIB anode and enables a theoretical capacity of 372 mAh·g⁻¹ for lithium storage via intercalation. Hard carbon is a SIB anode capable of a reversible capacity of >300 math g⁻¹. Subsequently, other SIB anodes have been developed, which can be categorized into three classes in terms of their electrochemistry: intercalation (de-intercalation)—(e.g., reduced graphene oxide, TiO₂, and Na₂Ti₃O₇), alloy (de-alloy) (e.g., Sn, red P, and Ge), and conversion (e.g., CuO and SnS). Among potential SIB anodes, conversion-based materials, such as metal oxides and metal sulfides, enable high specific capacity and cost-effectiveness. For example, copper(I) sulfide (Cu₂S) is a promising conversion material, having a large theoretical capacity of 337 mAh·g⁻¹ and low conversion potential of ˜0.4 V vs Na/Na⁺. In addition, Cu₂S below 104° C. (called as low-chalcocite) has the crystal structure of hexagonal close-packing lattice structure with Cu—S bond length of 2.18 Å. The large Cu—S bond length may facilitate the fast diffusion for Na-ions.

Previously some studies have been conducted for investigating Cu₂S as anodes in SIBs and examined the electrochemical behavior of a commercial micro-sized Cu₂S in SIBs. The large micro-sized Cu₂S powders were ball-milled to reduce their sizes and then mixed with SuperP conductive carbon and polyethylene oxide (PEO, a binder) in a 6:2:2 weight ratio. The micro-Cu₂S electrodes used Al foil as their current collectors and the fabricated electrodes were electrochemically evaluated in a voltage range of 0.4-2.6 V using the electrolyte of 1M NaCF₃SO₃ in tetra ethylene glycol dimethyl ether (TEGDME) solvent, but could only realize a low capacity of 200 mAh·g⁻¹ after 20 cycles at a current density of 50 mA·g⁻¹. Subsequently, a few more studies have improved the performance of Cu₂S electrodes using different strategies. Cu₂S electrodes have also been prepared by manually mixing a commercial micro-sized Cu₂S powder with SuperP and polyvinylidene difluoride (PVDF, a binder) in an 8:1:1 weight ratio. The Cu₂S electrodes used Cu foil as a current collector. Electrochemical testing was performed in the range of 0.2-2.5 V using the electrolyte of 1M NaPF₆ in diethylene glycol dimethyl ether (DEGDME) solvent. The Cu₂S electrodes showed dramatically improved stability, accounting for a sustainable discharge capacity of 270 mAh·g⁻¹ after 400 cycles and a Coulombic efficiency of ˜100% at 1 C (1 C=337 mA·g⁻¹). In comparison with DEGDME, a carbonate solvent (i.e., 1:1 ethylene carbonate/diethyl carbonate by volume) remarkably worsened the cycling ability of Cu₂S electrodes. A potential reason might be the formation of a protective layer of solid-electrolyte interphase (SEI) when using ether solvents and Na salts. Thus, it is significant to constitute a stable interface between a Cu₂S electrode and an ether-based electrolyte. A Cu₂S composite was also synthesized using a simple in-situ polymerization process and a subsequent carbonization process, in which Cu₂S nanorods were wrapped by N,S co-doped carbon (NSC), i.e., Cu₂S@NSC. The received Cu₂S@NSC composite was mixed with acetylene black and PVDF in a weight ratio of 70:20:10 for making electrodes on copper foil. Using an electrolyte that 1M NaClO₄ was dissolved in 1:1:1 dimethyl carbonate (DMC)/ethylene methyl carbonate (EMC)/ethylene carbonate (EC), it was found that, in the voltage range of 0.01-3.0 V, the Cu₂S@NSC composite electrode can only deliver a reversible capacity of 182.3 mAh·g⁻¹ after 50 cycles. Encouragingly, another study developed a type of Cu₂S micro-sized flakes with small carbon content using an electrochemical process. Mixing the Cu₂S flakes with Ketjen black and 1:1 poly(acrylic acid)/carboxyl methyl cellulose in a weight ratio of 7:2:1 in deionized water, the Cu₂S electrodes were fabricated and then tested in the electrolyte of 1M NaPF₆ dissolved in ethylene glycol dimethyl ether in the voltage range of 0.01-2.5 V. Results showed that the Cu₂S electrodes enable a superior rate capability (337 mAh·g⁻¹ capacity at 20 A·g⁻¹) and ultralong cycle life (88.2% of capacity retention after 5000 cycles at 5 A/g).

In addition to Cu₂S, some similar metal sulfides (e.g., Cu₉S₅ and Cu_(1.8)S) have also been investigated in SIBs. Using a one-step thermal treatment, a nanocomposite of Cu₉S₅/NSC was developed, in which nano-sized Cu₉S₅ nanoparticles (29 nm in average) were wrapped by amorphous NSC. Thermogravimetric analysis (TGA) showed that there are 64.3 wt % of Cu₉S₅ in the composite. The Cu₉S₅@NSC composite was further mixed with SuperP and carboxymethyl cellulose (CMC) binder at a 75:10:15 weight ratio to prepare electrodes on copper foil. Electrochemical measurements showed that the Cu₉S₅@NSC composite can maintain a reversible charge capacity of 344.3 mAh·g⁻¹ after 200 cycles at 100 mA·g⁻¹, accounting for an 83.6% retention rate compared to the first charge capacity of 412 mAh·g⁻¹. In addition, the composite enabled a capacity of 138.2 mAh·g⁻¹ at a high current density of 1600 mA·g⁻¹. Another work synthesized a hollow octahedral structure of digenite Cu_(1.8)S. The Cu_(1.8)S hollow particles as active material were mixed with acetylene black and a binder in a weight ratio of 80:15:5 for fabricating electrodes on Cu foil. Two electrolytes were used, 1 M NaCF₃SO₃ in EC and DMC, and 1 M NaCF₃SO₃ in DEGDME. The Na/Cu_(1.8)S half cells were cycled at different cutoff voltages (discharge to 0.7/0.5/0.1 V and charge to 1.5/2.2V). It was found that the ether-based DEGDME has good compatibility with Cu_(1.8)S. It was also revealed that, in the range of 0.1-2.2 V, the hollow Cu_(1.8)S hollow octahedra anode can only sustain 75% of its initial capacity after 50 cycles at 0.2 C. In comparison, the anode performance has been dramatically improved in the ranges of 0.5-2.2 and 0.7-2.2 V, accounting for ˜100% Coulombic efficiency. At a current density of 420 mA·g⁻¹, a capacity of ˜250 mAh·g⁻¹ and a superior cyclability over 100% retention were received after 1000 cycles in the range of 0.5-2.2 V. Similarly, a bullet-like Cu₉S₅ hollow particles coated with nitrogen-doped carbon (NC) have been reported. The resultant Cu₉S₅@NC composite as active material was mixed with SuperP and CMC in a weight ratio of 70:20:10 for fabricating electrodes. Using 1M NaCF₃SO₃ in DEGDME as the electrolyte and a voltage range of 0.4-2.6 V, the Cu₉S₅@NC composite electrode enabled stable cycling performance for a sustainable capacity of ˜300 mAh·g⁻¹ over 500 cycles at 0.3 A·g⁻¹, and a capacity retention of 79% after 4000 cycles at 2 A·g⁻¹. In another work, a hyperbranched polymer was used as a template and electrode additive for constructing hierarchical Cu₉S₅ nanoplates. The resultant Cu₉S₅ composite was mixed with SuperP and CMC in a weight ratio of 70:20:10 for making electrodes. The electrodes were tested in an electrolyte of 1M in 1:4 dioxlane (DOL)/dimethoethane (DME) by volume in a voltage range of 0.3-3.2 V. The composite electrode enabled high reversible capacity (429 mAh·g⁻¹ at 100 mA·g⁻¹), high-rate capability (300 mAh·g⁻¹ at 20 A·g⁻¹), and long-term stability (82.2% capacity retention over 1000 cycles).

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides high-performance Cu₂S anodes that provide a stable interface between Cu₂S anode and electrolyte.

In another embodiment, the present invention provides a ball-milling method to fabricate nanostructured Cu₂S wrapped by ultra-thin nitrogen-doped graphene sheets (NGS) for mass production. The resultant Cu₂S/NGS composite electrodes exhibited superior rate capability and long-term stable cyclability.

In yet other embodiments of the present invention, NGS is used as both a conductive agent and an encapsulation matrix.

In other embodiments, the present invention provides an electrode that is a composite of Cu₂S encapsulated by NGS, i.e. Cu₂S/NGS, that achieves a sustainable high capacity of 300 mAh g⁻¹ over 500 cycles and operates at high current rates up to 10 C (1 C=337 mA/g).

In other embodiments, the present invention provides an ultrathin metal oxide coating (e.g., Al₂O₃) via atomic layer deposition (ALD) to the composite to further boost the performance of the Cu₂S/NGS electrode.

In yet other aspects, the present invention provides a system and method for making superior Cu₂S anodes for sodium-ion batteries. The embodiment is cost-effective and feasible for industrial commercialization.

In yet other embodiments, the present invention provides sodium-ion batteries for energy storage applications such as for portable electronics, electric vehicles, and smart grids.

In another embodiment, the present invention provides an anode material, a current collector, a graphene-based material, and the graphene-based material covers the anode material. In other aspects, the graphene-based material is nitrogen-doped graphene that either partially or fully covers the anode material.

In another embodiment, the graphene-based material includes a plurality of channels that promotes ion transfer.

In another embodiment, the graphene-based material includes a plurality of doping sites made of at least one elemental dopant that form channels that promote ion transfer.

In another embodiment, the graphene-based material includes a plurality of N-doping sites that form channels that promote ion transfer.

In another embodiment, the elemental dopants are N, S, B, and P, or combinations thereof.

In another embodiment, the graphene-based material prevents polysulfide from contacting an electrolyte.

In another embodiment, a metal oxide is coated on the graphene-based material.

In another embodiment, the nitrogen-doped graphene prevents polysulfide from contacting an electrolyte.

In another embodiment, the present invention includes a metal oxide on the nitrogen-doped graphene. In a preferred embodiment, the metal oxide is Al₂O₃. In yet another preferred embodiment, the nitrogen-doped graphene is in the form of a sheet.

Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.

FIG. 1A shows an as-received Cu₂S powder has a particle size ranging from 10 to 50 μm for an embodiment of the present invention.

FIG. 1B is an SEM image of the electrode made from ball-milled Cu₂S and SuperP (indicated as Cu₂S/SuperP), showing significantly reduced particles with sizes in the nanoscale for an embodiment of the present invention.

FIG. 1C is an image of an electrode from ball-milled Cu₂S and NGS (i.e., Cu₂S/NGS), showing reduced particles with sizes mostly less than 2 μm for an embodiment of the present invention.

FIG. 1D shows an open-structure Cu₂S@SuperP electrode where Cu₂S particles are exposed to electrolyte for an embodiment of the present invention.

FIG. 1E shows a close-structure Cu₂S@NG electrode where Cu₂S particles are encapsulated by NG and isolated from the direct contact with electrolyte for an embodiment of the present invention.

FIG. 2A show a specific discharge capacity and Coulombic efficiency vs. cycles of Cu₂S/SuperP and Cu₂S/NGS electrodes in Na half cells tested under 0.01-3 V at a rate of 100 mA·g⁻¹.

FIG. 2B shows cyclic voltammetric profiles of a 5-cycle Na—Cu₂S/SuperP cell

FIG. 2C shows cyclic voltammetric profiles of an 11-cycle Na—Cu₂S/NGS cell.

FIG. 2D is a Nyquist plot from EIS measurements of a Na—Cu₂S/SuperP cell during 5 cycles.

FIG. 2E is a Nyquist plot from EIS measurements and a Na—Cu₂S/NGS cell during 50 cycles.

FIG. 3A is an SEM of the morphology of pristine Cu₂S/SuperP.

FIG. 3B is an SEM of the morphology of the Cu₂S/SuperP shown in FIG. 3A after 1^(st) discharge.

FIG. 3C is an SEM of the morphology of morphology of the Cu₂S/SuperP shown in FIG. 3A after 1^(st) charge.

FIG. 3D is an SEM of the morphology of pristine Cu₂S/NGS electrodes.

FIG. 3E is an SEM of the morphology of the Cu₂S/NGS electrodes shown in FIG. 3D after 1st discharge.

FIG. 3F is an SEM of the morphology of morphology of the Cu₂S/NGS electrodes shown in FIG. 3D after 1st charge.

FIG. 4A shows selected voltage plates on galvanostatic profile at 50 mA·g⁻¹ in 0.01-3.0 V for ex-situ XRD tests.

FIG. 4B show ex situ XRD patterns of pristine Cu₂S/NGS electrode and the electrodes after cycling at the potentials of 0.7 V (discharge), 0.3 V (discharge), 0.1 V (discharge), 0.01 V (discharge), 2 V (charge), and 3 V (charge) versus Na/Na⁺.

FIG. 4C is a comparison of the normalized Cu K-edge XAS spectra for Cu₂S Cu₂S/NGS electrodes discharged to 0.01V and charged to 3V. The dot-dash lines show the near-edge spectra of Cu and Cu₂S powder as references for Cu and Cu⁺ valency state. The inset shows the radical distance to first neighboring atoms.

FIGS. 5A, 5B, 5C and 5D are comparisons between Cu₂S/SuperP and Cu₂S/NGS electrodes for (a) (c) cycle performance with cutoff voltages of 0.01, 0.2, 0.4 V versus Na/Na⁺ at 100 mA·g⁻¹ and (b) (d) rate performance with a cutoff voltage of 0.2 V versus Na/Na⁺ at current densities of 100 mA·g⁻¹, 0.5, 1, 2, 5, and 10 C, respectively.

FIGS. 6A, 6B, 6C, 6D, 6E and 6F are Nyquist plots from EIS measurements during 50 cycles of a Na—Cu₂S/SuperP cell under (a) 0.2-3 V and (b) 0.4-3 V versus Na/Na⁺. (c) R_(SEI) film resistance in Na—Cu₂S/SuperP cell shows as functions of either cutoff voltage or cycle number. Nyquist plot from EIS measurements during 50 cycles of a Na—Cu₂S/NGS cell under (d) 0.2-3 V and (e) 0.4-3 V versus Na/Na⁺. (f) R_(SEI) film resistance in Na—Cu₂S/NGS cell shows as functions of either cutoff voltage or cycle number.

FIGS. 7A, 7B, 7C, 7D, 7E and 7F are comparisons of specific discharge capacity vs. cycles for pristine Cu₂S/NGS electrodes and the electrodes with ALD Al₂O₃ coating of 1, 2, 4, 6, and 8 nm. Nyquist plot from EIS measurements during 50 cycles of a Na-ALD/Cu₂S/NGS cell with the Al₂O₃ coating of (b 1 nm, (c) 2 nm, (d) 4 nm, (e) 6 nm, and (f) 8 nm at the cutoff voltage of 0.01-3 V versus Na/Na⁺.

DETAILED DESCRIPTION OF THE INVENTION

Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure, or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.

In a preferred embodiment, the present invention concerns systems and methods for creating anodes for use with sodium-ion batteries. In another preferred embodiment, the present invention provides a method of ball milling to accomplish superior Cu₂S anodes for sodium-ion batteries.

First, ball milling may be used to grind commercial micro-sized Cu₂S powders and mixing the powders with conductive agents and binders. Second, nitrogen-doped graphene (NGS) is used to replace traditional carbon black as conductive additive. This embodiment has multiple benefits such as suppressing formation of solid electrolyte interphase, inhibiting sulfur shuttling, and improving conductivity.

In other embodiments, atomic layer deposition may be used to improve the performance of Cu₂S anodes.

In yet other embodiments, the Cu₂S anodes for sodium-ion batteries (SIBs) may be further improved through coupling with a SIB cathode.

Using a ball-milling method to fabricate nanostructured Cu₂S wrapped by ultra-thin nitrogen-graphene sheets (NGS) produces Cu₂S/NGS composite electrodes that exhibit superior rate capability and long-term stable cyclability in comparison with Cu₂S/SuperP electrodes. Moreover, a uniform and conformal Al₂O₃ coating via atomic layer deposition (ALD) further improves the performance of Cu₂S/NGS electrodes.

In one embodiment, Copper(I) sulfide (Cu₂S) was mixed with a conductive additive (either SuperP or NGS (ACS Material)) and a PVDF binder (MTI Corporation) in 1-Methyl-2-pyrrolidone (NMP, Sigma-Aldrich) solvent in a weight ratio of 8:1:1. The mixtures were then ball-milled at 870 rpm for 6 hours using a planetary system (MSE Supplies LLC.) to grind down the Cu₂S particle size. The resultant slurries were subsequently cast on Cu foil with a thickness of 200 μm by a doctor blade. The electrode films were dried in an ambient environment and were heated at 100° C. in a vacuum for 8 hours. Al₂O₃ coatings were performed using atomic layer deposition (ALD, Savannah 200, Veeco) for different thicknesses at 100° C. The ALD precursors are trimethylaluminum (TMA) and H₂O.

Electrochemical Tests

The electrochemical performance of Cu₂S electrodes made in accordance with the present invention were evaluated in half coin cells (CR2032 stainless steel coin cells), using sodium metal as the counter electrode. Cu₂S electrodes were punched into circular disks with a 7/16-inch diameter, and the loading of Cu₂S active material is in the range of 1.5-2 mg cm⁻². Celgard polypropylene/polyethylene membranes (25 μm thick, MTI Corporation) were used as separators. Sodium metal (Millipore Sigma) was roll-pressed to sodium foil and then punched into 7/16-inch circular disks as counter electrodes. The liquid electrolyte was 1 M NaPF₆ (Sigma-Aldrich) in DEGDME (Sigma-Aldrich). The coin cells were then assembled and pressed under a hand-operated hydraulic crimping machine (MTI Corporation) at 1000 psi within an Ar-filled glove box with H₂O and 02 levels <0.01 ppm.

Galvanostatic cycling tests were performed on a Neware Battery Testing System at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were both performed on an SP-200 potentiostat (Bio-logic). CV applied a scan rate of 0.1 mV s⁻¹ in the voltage range of 0.01-3 V vs. Na/Na⁺. EIS were measured at the frequency range from 0.01 Hz to 100 kHz with an AC signal amplitude of 5 mV.

Characterizations

Materials morphologies were characterized using a scanning electron microscopy (SEM, NanoSEM 450). Elemental mapping of elements was conducted using energy-dispersive X-ray (EDX, NanoSEM 450) analysis. The phase structure and valence state of materials were determined using synchrotron-based X-ray diffraction (XRD, 11 ID-D beamline) and X-ray adsorption spectroscopy (XAS, 20 BM-B beamline) at Advanced Photon Source (APS) at Argonne National Laboratory, respectively. Ex-situ XRD and ex-situ XAS characterizations were performed at the beamlines 11 ID-D (0.1173 Å wavelength) and 20 BM-B (Cu K edge) at APS, respectively.

FIG. 1A shows that the as-received Cu₂S powder has a particle size ranging from 10 to 50 μm. In order to facilitate Na-ion diffusion into Cu₂S lattice during electrochemical cycling, the micro-sized Cu₂S particles were ball-milled at 870 rpm for 6 hrs. FIG. 1B is the SEM image of the electrode made from ball-milled Cu₂S and SuperP (indicated as Cu₂S/SuperP), showing significantly reduced particles with sizes in the nanoscale. In comparison, FIG. 1C is the image of the electrode from ball-milled Cu₂S and NGS (i.e., Cu₂S/NGS), showing reduced particles with sizes mostly less than 2 μm. The bigger particle sizes in Cu₂S/NGS electrodes are believed due to the porous structure of NGS and its high mechanical strength against the particle grinding. It is also noteworthy that the two electrodes have distinct morphologies. As illustrated in FIG. 1B, the Cu₂S/SuperP electrode has numerous cracks and voids fully accessible to the electrolyte.

In another aspect, the present invention provides an open-structure Cu₂S@SuperP electrode 100 on Cu foil 101 comprised of Cu₂S particles 102A-102H and SuperP particles 103A-103 as shown in FIG. 1D. As shown are SEI 104A-104C, Na⁺ diffusion 105A-105C and polysulfide shuffling 106A-106C.

In comparison, FIG. 1E shows a close-structure anode comprising an anode material which may be Cu₂S particles 202A-202E. Also provide is a current collector 201 which may be a copper foil. The embodiment further includes a graphene-based material 204A-204H. The graphene-based material covers the anode material.

In other aspects, the graphene-based material either partially or fully covers the anode material. In another embodiment, the graphene-based material prevents polysulfide from contacting electrolyte 206.

In yet other embodiments, the graphene-based material includes a plurality of channels that promotes ion transfer. In another embodiment, the graphene-based material includes a plurality of doping sites made of at least one elemental dopant that forms channels that promote ion transfer. The elemental dopants may be N, S, B, and P, or combinations thereof. FIG. 1E also shows SEI 207A-207C, Na⁺ diffusion 215A-21C and polysulfide shuffling 216A-216B.

The electrochemical behaviors of Cu₂S/SuperP and Cu₂S/NGS electrodes were comparatively investigated. FIG. 2A shows the cyclability and Coulombic efficiency (CE) of the electrodes at a current density of 100 mA g⁻¹ in the voltage range of 0.01-3 V. The Cu₂S/SuperP electrode faded in capacity and CE quickly in 10 cycles. CE here is the ratio of discharge capacity over charge capacity. The low CE (<50%) and the almost unchanged discharge capacity (˜300 mAh g⁻¹) for Cu₂S/SuperP electrode after 3 cycles reveal that there has an abnormally long charging phenomenon. This might have been caused by a polysulfide shuttling effect, as reported in Na—S batteries. In comparison, Cu₂S/NGS electrode delivered an initial discharge capacity of ˜450 mAh g⁻¹ and then >300 mAh g⁻¹ in 200 cycles with CE of 99%. Thus, the Cu₂S/NGS electrode has improved performance electrochemically in cyclability, efficiency, and sustainable high capacity. This also suggests that NGS replacing SuperP inhibits the polysulfide shuttling issue through isolating Cu₂S from direct contact with the electrolyte as shown in FIG. 1E. NGS is a physical barrier that reduces or inhibits S shuttling. The gradually fading capacity of Cu₂S/NGS after 80 cycles might be due to the reconstruction of Cu₂S/NGS during electrochemical cycling.

The CV profiles are shown in FIGS. 2B and 2C for the Cu₂S/SuperP and Cu₂S/NGS electrode, respectively. In the first discharge, both the electrodes show five reduction peaks at 0.41, 0.80, 1.16, 1.50, and 1.89 V commonly, ascribed to Cu₂S intercalation and conversion reactions. Two additional reduction peaks at low potentials of 0.13 and 0.01 V might be the side reactions between the electrodes and the electrolyte with the formation of solid electrolyte interphase (SEI). In the first charge, there are three common oxidation peaks at 1.59, 1.88, and 2.13 V for the two electrodes. After the first cycle, both electrodes underwent some distinct evolutions at each peak, indicating that electrochemical behaviors of the two electrodes are dynamically changing. In addition, regarding the polysulfide shutting issue in Cu₂S/SuperP electrode after 3 cycles, FIG. 2B shows many side reactions in the charge processes, likely related to polysulfide shuttling. In comparison, FIG. 2C shows fewer reactions during the charge processes.

The electrochemical stability was also investigated via EIS tests, the Cu₂S/SuperP cell shows a suddenly increasing impedance at the 5^(th) cycle (FIG. 2D), probably due to the unstable SEI formation and the polysulfide shuttling. Similarly, the Cu₂S/NGS cell has an increasing impedance in the first 5 cycles (FIG. 2E), probably due to the reconstruction of Cu₂S/NGS materials. However, the Cu₂S/NGS cell shows a decreasing impedance after 5 cycles, indicating some optimization by the reconstruction.

The morphological changes of the two Cu₂S electrodes during the first cycle were examined using SEM. Compared to the surface of a pristine electrode in FIG. 3A, the surface of Cu₂S/SuperP electrode (FIG. 3B) is covered by an additional dense layer after sodiation. This additional layer may be an SEI layer. In contrast, the sodiated Cu₂S/NGS electrode (FIG. 3E) has little difference with the pristine electrode (FIG. 3D), implying less SEI formation than that with the Cu₂S/SuperP electrode. After desodiation (FIG. 3F), the Cu₂S/NGS electrode almost recovers back to its pristine morphology (FIG. 3 a ), but the Cu₂S particles become smaller. The smaller Cu₂S particles provide direct evidence for the reconstruction of the Cu₂S/NGS electrode. In contrast, the desodiated Cu₂S/SuperP electrode (FIG. 3C) becomes more porous, compared to its pristine electrode in FIG. 3A. Compared to FIG. 3B, FIG. 3C implies that the SEI layer is unstable and can cause continuous electrolyte decomposition and increased impedance.

FIG. 4A shows the selected voltage for performing ex-situ synchrotron-based XRD analyses, including the voltages of 0.7, 0.3, 0.1, and 0.01 V during the discharge, and 2.0 and 3.0 V during the charge of the first cycle. XRD results in FIG. 4 b show that the pristine Cu₂S/NGS electrode contains mixed phases of Cu₂S, Cu_(1.8)S, and Cu_(1.92)S. Discharging to 0.7 V, the XRD patterns of Cu₂S phases became left-shifted, suggesting the enlarged volume of Cu₂S lattice by Na intercalation. Discharging to 0.3V, the shifted Cu₂S peaks have diminished in intensity, while simultaneously Cu and Na₂S peaks arise. This indicates that Cu₂S may have experienced a conversion reaction and converted into Na₂S and Cu between 0.7 and 0.3 voltage. Further discharging to 0.1 V, the Cu₂S features nearly have disappeared while Cu and Na₂S peaks become much evident. When fully discharging to 0.01 V, no significant phase changed for Cu and Na₂S patterns in comparison with that at 0.1 V. This may suggest that the electrochemistry of Cu₂S via sodiation may have accomplished above 0.01 V. On the other hand, the desodiated electrode at 2.0 V showed two major Cu and Na₂S patterns while some minor Cu_(x)S patterns. As desodation to 3.0 V, Cu₂S patterns are reversibly recovered and there is no Cu peak found. In addition, synchrotron-based XAS in FIG. 4C confirmed the reversible electrochemistry by tracking the valency state of the Cu element based on its K edge. The electrode discharged to 0.01 V exhibits similar Cu⁰ valency state and first-neighboring-atom distance to the standard Cu foil, suggesting the complete conversion from Cu₂S to Cu metal. As charging to 3 V, the valency state of Cu recovered to Cu¹⁺ with the close first-neighboring-atom distance to pristine Cu₂S powder. Thus, XRD and XAS strongly evidence that Na and Cu₂S undergo a highly reversible electrochemical reactions, including intercalation and conversion.

The ex-situ XRD results in FIG. 4B also confirmed two side reactions at 0.13 and 0.01 V, as illustrated in the CV profiles of FIG. 2C. The side reactions should be responsible for SEI formation. The stability of SEI is related to the performance of both the Cu₂S/SuperP and Cu₂S/NGS electrodes. A stable SEI is beneficial for achieving high performance of Cu₂S electrodes. Otherwise, an unstable SEI will degrade the performance of Cu₂S electrodes quickly. The SEI formation is always associated with electrolyte deposition and lithium consumption. The electrochemical performance (including cyclability, stability, rate capability and impedance) was investigated due to different discharge cutoff voltages (0.01, 0.2, and 0.4 V) on both the electrodes of Cu₂S/SuperP and Cu₂S/NGS. FIG. 5A shows the cyclability and CE of the Cu₂S/SuperP electrode, while FIG. 5C shows the cyclability and CE of the Cu₂S/NGS electrode. Apparently, the 0.2 V discharge cutoff voltage dramatically improved the cyclability and stability of Cu₂S/SuperP electrode (302 mAh g⁻¹ and 96% CE at 120 cycles) and Cu₂S/NGS (313 mAh g⁻¹ and 98% CE at 450 cycles). A higher discharge cutoff voltage further improved the cyclability and stability but sacrificed some capacity. EIS tests in FIGS. 6A through 6F also validated the SEI-suppressing effect of high discharge cutoff voltage on the SEI formation. The resistance of SEI layer calculated from those Nyquist plots exhibited a remarkable decrease as applying a cutoff voltage of >0.2 V. It validates that discharge cutoff voltage could significantly suppress the formation of an unstable SEI layer. In addition, rate capacity was also tested for both electrodes. FIGS. 5B and 5D show excellent capacity retention at high rates up to 10 C for both electrodes. However, FIG. 5B reveals that Cu₂S/SuperP electrode suffers some low CE at high rates of >5 C using a 0.2 V discharge cutoff voltage, possibly due to the polysulfide shutting issue. In comparison, the Cu₂S/NGS electrode exhibits an extremely stable high-rate performance (especially at 10 C) using a 0.2 V cutoff voltage, as illustrated in FIG. 5D.

In order to further suppress the formation of the SEI layer, as shown in FIG. 1E, a coating 300 may be applied to graphene-based material 204A-204G. The coating may be chemically inert to electrolyte and prevents the anode material from reacting with electrolyte. Coating 300 may be applied by ALD.

In a preferred embodiment, coating 300 is a metal or a metal oxide. In another preferred embodiment the metal oxide may be Al₂O₃ is Al₂O₃, ZrO₂, TiO₂, HfO₂ and combination thereof.

FIG. 7A compares the pristine Cu₂S/NGS electrode and the electrodes coated with Al₂O₃ coatings of 1, 2, 4, 6, and 8 nm in thickness. Apparently, Al₂O₃ coating thicker than 2 nm can effectively hinder the capacity fading of Cu₂S/NGS electrodes. Among all electrodes, the electrode with a 6-nm Al₂O₃ coating delivers the highest specific capacity after 100 cycles. However, a coating thicker than 6 nm resulted in a lower capacity than that of the electrode with a 6-nm thick coating. This may be due to some limited diffusion of Na⁺ to Cu₂S with a thick coating of 8 nm. In addition, the effect of ALD coating on the interfacial state was also examined by measuring EIS. Compared to the pristine electrode in FIG. 2 e , the impedances versus cycle with Al₂O₃ coatings were significantly reduced as the coating thickness thicker than 4 nm, as shown in FIG. 7 b-f . However, no more decrease in impedance when further increasing the coating thickness in FIG. 7D-7F. To summarize, a 6-nm thick Al₂O₃ coating via ALD shows the optimal performance in boosting the performance of the Cu₂S/NGS electrode.

The embodiments of the present invention deploy a technical route via ball milling to make superior Cu₂S anodes, in which NGS is used to replace SuperP as the conductive agents. In addition, NGS may serve as a physical barrier to inhibit S shuttling and SEI formation. Compared to the NGS/SuperP electrode, the Cu₂S/NGS electrode enables superior performance, including long-term cyclability, high efficiency, and high sustainable capacity. This may be ascribed to the multiple roles played by NGS. A cutoff voltage at 0.2 V also helped Cu₂S/NGS electrode to accomplish better performance, i.e., a sustainable capacity of ˜300 mAh g⁻¹ and 98% CE for over 450 cycles at 100 mA g⁻¹. It also helps to achieve high-rate capability up to 10 C (equivalent to 3.37 A g⁻¹). Moreover, a conformal and uniform Al₂O₃ coating via ALD further suppresses SEI formation and improves Cu₂S performance, enabling better electrochemical performance using the voltage window of 0.01-3 V.

While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above-described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure. 

What is claimed is:
 1. An anode comprising: a current collector; an anode material, said anode material is Cu₂S; a graphene-based material, said graphene-based material covers said anode material, and said graphene-based material includes a plurality of N-doping sites. 